The growth of most prostate cancers is driven by androgen receptor (AR), a transcriptional regulator that has both tumor-promoting and tumor-suppressive activities. Aggressive prostate cancer is treated through androgen deprivation therapy but eventually progresses to castration-refractory prostate cancer (CRPC), which is currently incurable. Potent AR antagonists and agents that inhibit androgen production eventually fail, as CRPC becomes androgen-independent, often due to the expression of constitutively active AR variants, notably AR-V7. New AR antagonists under development cause the degradation of AR and its androgen-independent variants but AR has not only a tumor-promoting but also a tumor-suppressive activity that will be abrogated by such drugs. Hence, novel classes of drugs that would be effective against androgen-independent CRPC are urgently needed. Towards this goal, we are targeting ?twin? kinases CDK8 and CDK19, which regulate transcriptional reprogramming, a key process required for cancer drug resistance and metastasis. CDK8 and CDK19 expression in clinical prostate cancers is strongly associated with CRPC and treatment failure. Senex Biotechnology has developed the first selective CDK8/19 inhibitors; the most recently identified lead compound shows excellent in vivo potency and pharmacokinetics. CDK8/19 inhibitors induce no apparent toxicity upon prolonged administration and show beneficial activities in different cancers, including growth inhibition of metastatic tumors. CDK8/19 inhibitors, when combined with anti-androgen therapy, suppress CRPC growth in vivo, with the strongest effect observed in an AR-V7 expressing CRPC model. The combinatorial effect of CDK8/19 inhibition and anti-androgen therapy is associated with changes in gene expression both in the tumor and in the stroma. The goals of the proposed Phase II SBIR program are to identify molecular characteristics of CRPC that make them susceptible to CDK8/19 inhibition, to determine if CDK8/19 inhibitors are effective against CRPC growing at metastatic sites, and to optimize the formulation of the lead CDK8/19 inhibitor. To achieve these goals, we will screen a panel of cell-line based and patient-derived xenograft CRPC models for in vivo response to CDK8/19 inhibition combined with castration or enzalutamide. The observed responses will be correlated with the tumor genomics and gene expression before and after treatment, to identify molecular determinants of sensitivity to CDK8/19 inhibitor combined with anti-androgen therapy. We will also evaluate the effects of the lead CDK8/19 inhibitor on CRPC growth and metastatic spread after orthotopic implantation and on CRPC growth in the bone, the primary metastatic site in the clinic. Concurrently, we will optimize the formulation of the lead CDK8/19 inhibitor candidate to achieve the best pharmacokinetics. Upon completion of this program, the CDK8/19 inhibitor drug candidate will be ready for IND-enabling studies, and the generated information will be used to guide patient selection and the design of clinical trials.